A Zion police officer will not face criminal charges for fatally shooting a 17-year-old in the back, an incident that has fueled public anger and highlighted tension between Lake County authorities and some minority residents.

Revealing details that had been obscured since police shot Justus Howell on April 4, State's Attorney Mike Nerheim announced Thursday that he had decided the officer's use of force was justified. Nerheim, who displayed low-resolution video of the shooting at a news conference, said investigators determined that Howell had a gun in his hand and turned toward the officer just before he was shot.

Controversy over the white officer's shooting of a black teenager has been fed by circumstances specific to the case and grievances over police use of force against minorities in recent incidents across the country. Thursday's airing of the evidence did not settle disagreements over Howell's death.

Activists called for protests, the family's attorney promised a federal lawsuit and the teen's mother, LaToya Howell, said she was unconvinced by the evidence, including grainy footage of the shooting caught by a surveillance camera.

"I have seen that video. There is nothing that suggests they should execute my son. His back was turned. He was no threat," she said outside the courthouse. "This is not proper policing. This is not the end. Justice for Justus."

For the first time, Nerheim publicly identified the officer who shot Howell as Eric Hill, a nine-year veteran of the department, according to state records. He remained on paid leave Thursday but was expected to return soon, authorities said. Hill could not be reached for comment.

At the news conference, Nerheim pointed to video footage as he walked the media through an investigation that has stretched more than a month. At the request of local authorities, the FBI monitored the investigation, which was conducted by the Lake County Major Crime Task Force.

"Officer Hill provided Howell ample opportunity to drop the weapon, and only fired when he felt that his life and the life of his fellow officer was in danger. These are the facts. Justus Howell was armed and dangerous. For those reasons Officer Hill was in fear for his life, and acted reasonably and justifiably to protect himself and his fellow officer," said Lake County State's Attorney Michael Nerheim on May 14, 2015.

"Officer Hill provided Howell ample opportunity to drop the weapon, and only fired when he felt that his life and the life of his fellow officer was in danger. These are the facts. Justus Howell was armed and dangerous. For those reasons Officer Hill was in fear for his life, and acted reasonably and justifiably to protect himself and his fellow officer," said Lake County State's Attorney Michael Nerheim on May 14, 2015.

The incident started when Howell, who lived in Waukegan, arranged to meet a man in Zion to buy a semi-automatic pistol for $600, according to Nerheim. Howell tried to steal the gun instead, Nerheim said, and the prosecutor displayed separate surveillance camera video that appeared to show Howell and the man tussling. Howell pointed the gun at the man, who pushed the weapon toward the ground, and it discharged, authorities said.

Officers arrived to see Howell fleeing, the prosecutor said. Hill chased Howell through yards and ordered him to stop, Nerheim said. The officer saw a gun in Howell's hand, an observation echoed by witnesses, authorities said. Howell was running in the direction of another officer, according to Nerheim.

Near 24th Street and Galilee Avenue, the officer saw Howell turn left toward him with the gun in his right hand, Nerheim said. Fearing he would be shot, Hill fired twice in quick succession, the prosecutor said.

After the shooting, information Nerheim called inaccurate proliferated among the public and in some media. A key question centered on whether Howell dropped the gun before he was shot. Nerheim said Howell was still holding the weapon when he was shot, and investigators found the loaded gun within a foot of Howell's head.

Nerheim acknowledged it was difficult to make out what happened in the video, though he voiced certainty that Howell had turned toward the officer. Nerheim also cited an analysis of the bullet wounds that indicated Howell turned toward Hill.

Hill gave Howell ample opportunity to drop the gun and fired as a last resort when he feared for his life and the lives of others, Nerheim said.

"My deepest condolences to the family of Justus Howell. This decision in no way takes away from the fact that they're suffering from an incredible loss," he said.

The man who allegedly arranged to sell Howell the gun, Tramond Peet, 18, of Lindenhurst, is charged with aggravated unlawful use of a weapon and is free on bond, according to court and jail records. The gun, a Kimber 9 mm pistol, was stolen from a Lake Villa home a few days before the shooting, authorities said.

Prosecutors have authority over whether to charge an officer, though Illinois, like other states, shields police from prosecution under many circumstances. A police shooting can be legal regardless of whether it was necessary, and officers can be legally justified in shooting people in the back.

Illinois law allows an officer to shoot a fleeing person if the officer reasonably believes the shooting is needed to prevent the escape of a person who either has committed or attempted a felony involving serious bodily harm, is using a deadly weapon to try to escape or would endanger human life unless immediately stopped.

An attorney for the Howell family, Matthew Dudley, of Libertyville, said he intends to file a federal lawsuit against the Zion police in the coming days. Like Howell's mother, Dudley said he doesn't think the video shows Howell turning toward the officer.

"It does not appear that Justus was making any turn. He was just fleeing," Dudley said.

"We don't want people out here tearing anything up or burning anything. We just want justice. We don't want a circus or anything like that. We just want to be treated right," he said.

"He didn't deserve to die," the uncle said.

Protests in the wake of Howell's shooting have focused both on his death and on more general complaints about law enforcement in Lake County's northern cities, where episodes of police violence against black and Hispanic residents have periodically sparked uproars. Police in those cities have more routinely faced brutality allegations than officers in the wealthier, whiter towns to the south.

Zion, a city of some 24,000 that is about 60 percent black and Hispanic, is patrolled by a police force of about 50 officers that includes three African-Americans and four Hispanics, according to recent state records. The far northern suburb is not unique in having a largely white police department and a mostly black and Latino population.

Zion police Chief Stephen Dumyahn said at the news conference that the city will review the case, though he said the police response appeared to have been appropriate.

Dumyahn said he thinks the department's relationship with most of the city's residents is "very strong." He said, however, that the department has explored outfitting officers with body cameras, devices that activists have called for in the wake of police uses of force.

Howell's shooting happened amid a wave of controversial uses of force by police nationwide — incidents followed by complaints about the rarity of criminal charges against police. Despite the lack of charges, Justus' mother, LaToya Howell, said she will continue to advocate on behalf of her late son and local youths.